U.S. patent number 6,277,097 [Application Number 09/045,314] was granted by the patent office on 2001-08-21 for injection system.
This patent grant is currently assigned to Novo Nordisk A/S. Invention is credited to Lars Peter Klitmose, Andre Larsen, Soren Mikkelsen.
United States Patent |
6,277,097 |
Mikkelsen , et al. |
August 21, 2001 |
Injection system
Abstract
An injection system for preparing a mixture of a solvent and a
medicament and for subsequent dosed injection of the mixture
comprises a syringe accommodating an ampoule (7) in which a liquid
is stored between a membrane (8) dosing one end of the ampoule (7)
and a piston (6) which can by a piston rod (3) be forced into the
ampoule (7) to press out a dose of the liquid. The piston rod (3)
and the piston (6) has mutual engaging threads (4, 5) by which the
piston (6) is coupled to the piston rod (3) to follow this rod in
both axial directions. An outer thread (30) engaging an inner
thread (29) in a housing is provided on a part (28) of the piston
rod (3) so that rotation of the piston rod (3) will screw this rod
and the piston (6) into the ampoule (7). A detent (18) ensures that
the piston rod (3) is only rotated in a dose administering
direction. However, the thread (30) of the piston rod (3) can be
disengaged from the thread (29) of the housing to set the piston
rod (3) free to be moved in a proximal direction.
Inventors: |
Mikkelsen; Soren (Ballerup,
DK), Klitmose; Lars Peter (Gentofte, DK),
Larsen; Andre (Drag.o slashed.r, DK) |
Assignee: |
Novo Nordisk A/S (Bagsvaerd,
DK)
|
Family
ID: |
26063815 |
Appl.
No.: |
09/045,314 |
Filed: |
March 20, 1998 |
Foreign Application Priority Data
|
|
|
|
|
Mar 25, 1997 [DK] |
|
|
0339/97 |
May 5, 1997 [DK] |
|
|
0513/97 |
|
Current U.S.
Class: |
604/187;
604/195 |
Current CPC
Class: |
A61M
5/24 (20130101); A61M 5/31553 (20130101); A61M
5/31586 (20130101); A61J 1/2096 (20130101); A61M
5/2448 (20130101); A61M 5/3156 (20130101); A61M
5/3202 (20130101); A61M 2005/2411 (20130101); A61M
2205/581 (20130101); A61M 2205/582 (20130101); A61J
1/201 (20150501); A61J 1/2065 (20150501) |
Current International
Class: |
A61M
5/24 (20060101); A61M 5/315 (20060101); A61J
1/00 (20060101); A61M 5/32 (20060101); A61M
005/00 () |
Field of
Search: |
;604/186,200,201,202,206-211,232,195,187,415,82,87 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nguyen; Anhtuan T.
Assistant Examiner: Sirmons; Kevin C.
Attorney, Agent or Firm: Skadden, Arps, Slate, Meagher &
Flom LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. 119 of Danish
applications 0339/97 filed Mar. 25, 1997 and 0513/98 filed May 5,
1997, and U.S. provisional application 60/047,930 filed May 30,
1997 the contents of which are fully incorporated herein by
reference.
Claims
What is claimed is:
1. An injection system for use in preparing a mixture of a solvent
and a medicament and for subsequent dosed injection of the mixture,
the system comprising a syringe having a housing with proximal and
distal ends, and a replaceable ampoule secured relative to the
syringe housing, wherein the ampoule includes a first end closed by
a pierceable membrane, a second end closed by a piston having a
first coupling element, and a solvent stored between the membrane
and piston; and wherein the syringe comprises:
means located at the distal end of the housing for mounting a
disposable needle;
a piston rod which is supported in the housing for axial movement
and which includes a second coupling element which detachably
couples the piston rod to the piston, such that the piston follows
movement of the piston rod in both axial directions;
a dose setting and injection mechanism supported by the housing for
setting doses and for advancing the piston rod in a distal
direction to inject a set dose;
a disengageable coupling between the housing and the piston rod to
allow movement of the piston rod in the distal direction by means
of the dose setting and injection mechanism to eject solvent from
the ampoule to mix with a medicament, but to prevent movement of
the piston rod in a proximal direction, when an ampoule is secured
to the syringe; and
a release mechanism which can be activated for selectively
releasing the disengageable coupling between the piston rod and the
housing while an ampoule is secured to the syringe to allow the
piston rod to move in the proximal direction to suck back a mixture
into the ampoule, and deactivated to re-engage the disengageable
coupling to make movement of the piston rod depend on the dose
setting and injection mechanism to prepare the system for injecting
such mixture.
2. A system according to claim 1, wherein the first and second
coupling elements comprise threads.
3. A system according to claim 1, wherein the syringe housing
includes a stop, wherein the dose setting and injection mechanism
includes a member which abuts the stop to limit the movement of the
piston rod in the distal direction, wherein the member can be
rotated in a first rotational direction away the stop to set a
dose, the angle of rotation being proportional to the dose set, and
wherein the member, to inject the dose, can be rotated in a second
rotational direction back into engagement with the stop.
4. A system according to claim 1, wherein the syringe housing has
an inner thread, wherein the disengeable coupling includes a driver
element having an outer thread which selectively engages the inner
thread, and a unidirectional coupling between the dose setting and
injection member and the piston rod which coupling transmits
rotation of the dose setting and injection member to the piston rod
only when the member rotates in the second rotational direction,
wherein the driver element, responsive to being rotated in the
second rotational direction, moves the piston rod in the distal
direction, and wherein the release mechanism is coupled to the
driver element to allow the thread of the driver element to be
drawn out of engagement with the inner thread of the syringe
housing to allow the piston rod to be moved in a proximal
direction.
5. A method of preparing a mixture of a solvent and a medicament
and for subsequent dosed injection of the mixture, comprising the
steps of:
providing a syringe having a housing with proximal and distal ends,
a piston rod which is supported in the housing for axial movement,
a dose setting and injection mechanism supported by the housing for
setting doses and for advancing the piston rod in a distal
direction to inject a set dose, a disengageable coupling between
the housing and the piston rod to allow movement of the piston rod
in the distal direction, but to prevent movement of the piston rod
in the proximal direction, when an ampoule is secured to the
syringe, and a release mechanism for selectively releasing the
disengageable coupling between the piston rod and the housing,
while an ampoule is secured to the syringe, to allow the piston rod
to move in the proximal direction;
securing a replaceable ampoule relative to the syringe housing,
wherein the ampoule includes a first end closed by a pierceable
membrane, a second end closed by a piston having a first coupling
element, and a solvent stored between the membrane and piston;
detachably coupling the first coupling element to a second coupling
element on the piston rod, such that the piston follows movement of
the piston rod in both axial directions;
ejecting the solvent from the ampoule using the dose setting and
injection mechanism;
mixing the ejected solvent with a medicament; and
sucking the mixed solvent and medicament back into the ampoule by
actuating the release mechanism and moving the piston rod in a
proximal direction.
6. A method according to claim 5, wherein the first and second
coupling elements are threads and the coupling step is performed by
screwing the coupling elements together.
Description
The invention relates to an injection system for preparing a
mixture of a solvent and a medicament and for subsequent dosed
injection of the mixture, the system comprising:
a syringe from which set doses are apportioned from a cylinder
ampoule having a first end closed by a pierceable membrane and a
second end closed by a piston which can by a piston rod, which is
successively advanced in a distal direction by the injection of set
doses, be forced into the ampoule to press out a dose of a
medicament stored in the ampoule between the membrane and the
piston through an injection needle piercing the rubber
membrane,
an ampoule with a solvent fitting into the syringe, and
a needle mounted in a needle hub and having an injection part
projecting from a distal side of the hub and a back needle piercing
the membrane of an ampoule when the needle hub is mounted on the
syringe.
Whereas many medicaments are supplied as a solution or a suspension
in pre-filled cylinder ampoules of the kind mentioned above, some
kinds of medicine have a low storability once they are dissolved.
Therefore these types of medicine are mainly stored in a vial as a
lyophilised powder which is dissolved in a solvent which is
injected in the vial by the user immediately before he is going to
load his syringe with a new ampoule. The solution obtained in the
vial is transmitted to a cylinder ampoule which is then ready to be
mounted in a syringe of the above mentioned kind.
By the mixing it is important that a precisely measured amount of
solvent is used for dissolving the powder in the vial as else the
concentration of the mixture in the ampoule will be wrong so that
the user loses control of his medication. A precise mixing may be
obtained by storing the solvent in the cylinder ampoule which is
going to be used for the mixture. From the manufacturer the ampoule
may contain a very precise amount of solvent which may be injected
into the vial through a double ended needle penetrating the closing
membranes of as well the ampoule as the vial.
When the powder in the vial is dissolved in the solvent injected in
the vial, mixture obtained is sucked back into the ampoule by
drawing the piston of this ampoule outward. If less than the total
amount of solvent was injected into the vial, the remaining solvent
will be mixed up in the mixture of medicine and solvent transmitted
from the vial back into the ampoule.
When the piston of the ampoule is drawn outward to suck up the
mixture in the ampoule it is very important that the piston is not
drawn out of the ampoule as it may be difficult to put back again
an as it will inevitably cause spill and contamination of the
medicament.
From WO 95/12425 is known a device for filling a cylinder ampoule
for use in a syringe of the above mentioned kind. This apparatus
comprises a holder in which an ampoule containing a measured amount
of solvent may be mounted. At its front end the holder is equipped
with a connecting device coupling the outlet end of the ampoule to
a vial into which the solvent shall be injected to dissolve a
lyophilised medicament. To the rear end of the holder a support is
mounted In which a piston rod may be moved in the axial direction
of the ampoule. The distal end of the piston rod is screwed into
the piston and the proximal end of the piston rod projects from
said support. The solvent in the ampoule may be pressed through a
needle which from the front end of the ampoule forms a channel to
the interior of the vial. The solvent is transmitted from the
ampoule to the vial by pressing the projecting end of the piston
rod which then presses the piston into the ampoule. The mixture
obtained in the vial is thereafter sucked back into the ampoule by
drawing the piston outward. This outward drawing is made possible
by the fact that a rear end of the piston is provided with a hole
with an inner thread which is engaged by an outer thread at the
distal end of the piston rod. The piston rod support which forms a
stop preventing the piston from being drawn out of the ampoule is
fixed to the ampoule holder in such a way that it cannot be unfixed
unless the screw connection between the piston and the piston rod
is brought out of engagement. This way the risk for inadvertently
drawing out the piston is eliminated.
However, the described solution implies that different devices must
be carried and used. First the filling device must be taken apart
and the ampoule mounted into said device. Then a filling procedure
must be run through. Thereafter the device must be taken apart
again to remove the filed ampoule, which must then be installed in
the syringe by which a dose may be set and injected.
Consequently it is an object of the invention to provide an
injection system which makes a special filling device
superfluous.
This is obtained by a system of the kind mentioned in the opening
of this application, which system is characterised in that the
piston rod of the syringe and the piston of the ampoule has
co-operating coupling means by which the piston of the ampoule is
secured to the piston rod of the syringe to make said piston follow
movements of the piston rod in both axial directions.
In such a syringe an ampoule with solvent may be mounted with the
distal end of the piston rod engaging the piston of the ampoule,
and through a needle mounted in the common way on the syringe the
solvent may be injected into a vial by repetitively setting a dose
and injecting this dose through the closing rubber membrane of the
vial until the ampoule is emptied. An adapter may be mounted
between the syringe and the vial to keep these elements in position
during said operation and during the next operation which is to
draw the piston rod in a proximal direction whereby it due to its
engagement with the piston in the ampoule will draw this piston
outward in the ampoule. Thereby the mixture prepared in the vial by
the injection of the solvent will be sucked into the ampoule from
where it may now be apportioned in doses which may be individually
set by the dose setting mechanism.
The coupling means of the piston rod and the piston may
appropriately be parts of which one carries an outer thread which
can engage an inner thread in the other part. However other kinds
couplings allowing transmission of axial movements in both axial
direction may be used so as snap couplings or bayonet couplings. In
some syringes the piston may be moved in both axial directions
against the resistance of friction in a piston rod guidance whereas
other syringes are provided with a unidirectional coupling which
allows only a movement of the piston rod and the piston in a distal
direction when an ampoule is mounted in the syringe. When the
ampoule is dismounted the unidirectional coupling is released and
the piston rod may be moved in the proximal direction to a
retracted position which it takes up when a full ampoule is
mounted. In such syringes means may be provided which even when an
ampoule is mounted in the syringe can be operated to set the piston
free to be moved in a proximal direction.
According to a preferred embodiment of the invention he dosing
mechanism of the syringe comprises a dose setting and injecting
member which to set a dose is rotated from a stop an angle which is
proportional with the set dose and which to inject the set dose is
rotated back to the stop, an inner thread in a tubular housing of
the syringe, an outer thread provided on elements forming a part of
a piston rod assembly, which outer thread is in engagement with the
inner thread of the housing, a unidirectional coupling between the
dose setting and injecting member which only by injecting transmits
the rotation to the piston rod to screw this rod in a distal
direction through the housing, which syringe may according to the
invention be characterised in that the means setting the piston rod
free are means drawing the elements carrying the external thread
out of engagement with the inner thread of the housing. In such a
syringe the means setting the piston rod free may be means drawing
the elements carrying the external thread of the piston rod out of
engagement with the inner thread of the housing.
An ampoule according to the invention is an ampoule designed to fit
into a syringe for apportioning of set doses and which ampoule has
a piston provided with coupling means designed for co-operation
with coupling means on the piston rod of said syringe to secure
said piston to said piston rod.
In the following the invention is described in further details with
reference to the drawing, wherein
FIG. 1 shows a sectional view of a syringe according to the
invention,
FIG. 2 shows in an enlarged scale the proximal part of the syringe
shown in FIG. 1.
FIG. 3 shows a perspective view of a carrier in the syringe
according to FIGS. 1 and 2,
FIG. 4 shows perspective view of a piston rod assembly for the
syringe shown in FIGS. 1 and 2,
FIG. 5 shows a perspective view of a piston rod driver for the
syringe shown in FIGS. 1 and 2,
FIG. 6 shows an expanded view of the piston rod assembly shown in
FIG. 4,
FIG. 7 shows a sectional view of the piston rod assembly, and
FIG. 8 shows the distal end of a syringe coupled to a vial.
The syringe in FIG. 1 has a tubular housing 1 with an inner tubular
housing 2. The distal ends of the end of the housings form a
cartridge holder and the proximal ends accommodate mechanisms by
which mixing of a medicament, dose setting and injection are
controlled.
A piston rod 3 is at its distal end provided with an outer thread 4
which fits into an inner thread 5 in a piston 6 of an ampoule 7
which is at its distal end closed by a membrane 8. By mounting of
the ampoule this ampoule is rotated so that the thread 4 of the
piston rod 3 engages the thread 5 of the piston in the ampoule 7. A
flange 9 near the distal end of the piston rod 3 defines the
distance the piston rod 3 may be screwed into the piston 6 so at
the piston rod 3 is not wedged in the piston 6. Her the connection
between piston and piston rod is shown as a thread connection but
other connections so as bayonet couplings may be used to secure the
piston to the piston rod in such a way that the piston will follow
the piston rod during axial movements thereof in both
directions.
When the piston 3 of the ampoule 7 is attached to the piston rod 3
the cartridge holder is closed by an end stopper 10 screwed on the
distal end of the inner housing 2. The membrane 8 of the ampoule is
sealed on this ampoule by a member having a threaded opening in
which a needle hub 11 with a double pointed needle 12 may be
received so that one end of the needle 12 penetrates the rubber
membrane 8 of the ampoule 7 whereas the other end of the needle may
be inserted through the skin of a person who shall receive an
injection. When the syringe is not in use the needle receiving part
with the needle may be covered by a cap 13 so that nobody is hurt
by the needle.
In its withdrawn position as shown in FIG. 1 the piston rod 3 is
accommodated in the proximal part of the inner housing where it is
surrounded by a carrier 14 which is shown in FIG. 3 and is a
longitudinal tubular member having three longitudinal slots 15 in
approximately its full length and a flange 16 at its distal end. At
its proximal end the carrier 14 has a part with a not round cross
section and which is snapped into a correspondingly not round
opening in a driver 17 through which rotation is transmitted to the
carrier during injection operation of the syringe as it will be
described be low. The carrier 14 may be rotated about its axis but
a ratchet 18 coupled between the carrier 14 and the inner housing 2
has the effect that the carrier is rotatable in only one
direction.
The proximal part of the housing 1 is surrounded by a cap 19 which
may rotate on said proximal part. An edge of the cap 19 is provided
with a dose indicating scale 20 which may by rotation of the cap be
moved relative to an arrow 21 on the housing 1 at the transition
between the proximal and the distal part. A protrusion 22 on outer
wall of the housing 1 provides a stop against which a not shown
protrusion in the cap abuts. When the cap is rotated in a direction
away from the stop a circular surface with saw tooth shaped steps
having a steep and an inclined edge provided on the inner side of
the end bottom 23 of the cap 19 slides over a corresponding
surfaced provided on the driver 17. The surfaces are forced against
each other by a spring 24 so that a click is heard each time the
top of the steps on one of the surfaces fall down in the bottom of
the steps on the other surface. When the cap is rotated back until
its protrusion abuts the stop 22 on the outer wall of the housing 1
the end bottom 23 will transmit the rotation to the driver 17 as
the steep edge of the steps on the circular surfaces of the end
bottom 23 will now abut the steep edges of the steps on the driver
17. The driver 17 is then rotated and is rotation is transmitted to
the carrier 14 the ratchet 18 being so oriented that it allows this
rotation.
FIG. 5 shows a piston rod driver 25 comprising a disc 26 from the
edge of which three 120.degree. displaced flexible arms 27 extend
in an axial direction. At the outer ends of the arms 27 members 28
are provided each carrying a thread 30 which may engage a matching
thread 29 on the inner side of the proximal part of the inner
housing 2 when the members 28 are supported so that the flexible
arms 27 do not bend away so that the threads 30 and 29 are
disengaged.
The piston rod 3 which at its distal end is provided with the tread
4 for engagement with a thread in the piston 6 of an ampoule is a
part of a piston rod assembly which is shown in FIGS. 4, 5 and 6 in
perspective, as an expanded view, and a sectional view
respectively. The piston rod 3 is tubular and is at its proximal
end provided with three axial, 120.degree. displaced slots dividing
the end of the piston rod into three tongues 31. At the bottom end
of each of the slots the piston rod is provided with a protrusion
32 which engages the slots 15 of the carrier 14. The piston rod
driver 25 is mounted with its disc 26 abutting the end of the
tongues 31, its arms 27 engaging the slots between the tongues 31,
and the free ends of its threaded members 28 abutting the proximal
ends of the protrusions 32. The protrusions 32 and the threaded
members 28 of the piston rod driver protrude through the slots 15
of the carrier and this way rotation of the carrier will be
transmitted to the piston rod 3 and the piston rod driver 25
whereas piston rod 3 and piston rod driver 25 may be axially
displaced relative to the carrier 14. A support member having
protrusions 33 engaging the slots between the tongues 31 of the
piston rod 3 is against the force of a spring 34 axially
displaceable in the piston rod 3 from a position near the distal
end of said slots to a position near the free end of the tongues
31. In the position near the bottom end of the slots the
protrusions 33 will support the members 28 and keep their threads
30 in engagement with the thread 29 of the housing, but when the
protrusion are moved to their position near the end of the tongues
31 the arms 27 of the piston rod driver is supported near the disc
26 and the members 28 may due to the oblique sides of the engaging
threads 29 and 30 be pressed inward toward the axis so that they
may be moved axially with their threads 30 scratching over the
thread 29 in the housing.
FIG. 6 shows an expanded view of the piston rod assembly. It is
seen that the piston rod assembly is provided by combining a
tubular part forming the actual piston rod 3 and provided with a
thread 4 the active length of which is limited by a flange 9
provided by a ring mounted on the piston rod. The protrusions 32
are provided on a part 35 added to form a part of the piston rod 3.
The same way the slotted part of the piston rod is added as an
individual part. Between the slotted part an the rest of the piston
rod a ring 35 is mounted forming an abutment for the spring 34
which is inserted between said ring and a flange on a tubular
sleeve 36 in which an end of the supporting element is secured. A
spindle 37 extends all the way through the piston rod, the
supporting element and the disc of the piston driver and has at its
distal end a head 39 having a diameter which is larger than the
diameter of the bore in the sleeve 36 so that this head will engage
the flanged end of the sleeve 38 if the spindle is drawn in the
proximal direction and will against the force of the spring 34 move
the supporting element on which the sleeve 36 is secured to a
position wherein its protrusions lies near the free ends of the
tongues 31. The spindle 37 further projects through the driver 17,
the end bottom 23 of the cap 19 and into a button 38. By a thread
39a the spindle is secured to the driver 17 and to the button 38
whereas the diameter of the opening through the end bottom 23 of
the cap 19 is large enough to let the spindle 37 pass freely.
Due to the ratchet 18 the carrier may only be rotated in a
direction by which the piston driver is rotated relative to the
housing in such a direction that said piston driver is screwed in a
direction towards the distal end of the syringe to press out the
content of an ampoule mounted in the syringe. A dose may therefore
be set by rotating the cap away from the stop 22 in the opposite
direction to make the coupling between the driver 17 and the end
bottom 23 of the cap 19 click over a number of teeth corresponding
to the size of the wanted dose this way giving an audible
indication of the size of the set dose. During the clicking the
button 6 will sink and jump back at each click to give a tactile
indication of the size of the set dose. Thereafter the set dose may
be injected by rotating the cap 19 back to abutment with the stop.
During this rotation the coupling between the end bottom of the cap
and the driver 17 will transmit the rotation to the driver 17 which
will again rotate the carrier 14 which transmits the rotation to
the piston driver to screw it along the housing 2 a distance
proportional with the angle of rotation.
When the syringe is going to be used for injection of a medicament
which has to be mixed from two or more components immediately
before the use, an ampoule with a solvent component is mounted in
the syringe, a needle is mounted and the free end of this needle is
inserted in a vial containing another possibly solid component of
the medicament. In FIG. 8 this insertion is illustrated by a vial
40 which is closed by a stopper 41 through which the needle 12 is
inserted. It is further illustrated how an adapter 42 may be placed
on the vial 40 which adapter has a socket to which the distal end
of the syringe may be mounted using the coupling which normally is
used for fixing the needle protection cap 13. With the needle
inserted in the vial repetitive dose setting and injection
rotations of the cap is performed whereby the piston is
successively moved into the ampoule until the full content thereof
is injected into the vial 40.
Hereafter the vial may be turned upside down still with the needle
inserted through the stopper of the vial. Now the piston in the
ampoule may be drawn back to its position corresponding to a full
ampoule whereby the mixture provided in the vial may be sucked back
into the ampoule. This is obtained by drawing the cap in a proximal
direction which means that the spindle 37 is pulled in this
direction. Hereby the driver which is secured to the spindle and
snap locked onto the carrier 14 will first be lifted off said
carrier where after the spindle may be moved freely until its head
39 abuts the sleeve 36. Further movement of the spindle 37 will
cause a compression of the spring 34 and the protrusions of the
support member will be moved to their position near the free end of
the tongues 31 where the piston threaded members 28 of the piston
driver 25 is no longer supported. The axial pull of the spindle is
now transmitted to the piston rod which is restrained in moving due
to the engagement between the inner thread in the housing and the
threaded members 28 of the piston drive. However, due to the
lacking support of the members 28 these may be bend inward towards
the axis and the engagement between the threads 29 and 30 may be
released so that the piston rod 3 may be drawn backward with the
piston drive members scratching over the thread 29 in the housing.
Due to the engagement between the thread 4 at the distal end of the
piston rod 3 and the thread in the piston 6 of the ampoule 7 said
piston is drawn outwards in the ampoule and the content from the
vial 40 is sucked over into the ampoule 7. When the spindle 37
thereafter is moved back to its original position the spring 34 is
released and the support member is moved back so that its
protrusions 33 support the threaded members of the piston rod
driver 25 to keep the tread of these members in engagement with the
inner thread 29 of the housing. Finally the driver 17 is snapped
onto the carrier 14 again and the syringe is ready for use as a
syringe by which a dose may be set and injected.
In the syringe described the piston rod is bound to be moved in the
distal direction by the dosing mechanism due to the ratchet 18
which provides a one way coupling which ensures that the piston rod
is only movable in said distal direction as long as it is coupled
to the dosing mechanism. In other types of syringes the piston rod
is prevented from being moved in a proximal direction by detent
mechanisms comprising a pawl which secured to the housing engages a
toothing along the piston rod or comprising a leaf spring which
secured to the housing cuts into the piston rod. To realise the
invention such a pawl engagement or cutting engagement may by
appropriate members be operated to be disengaged to set the piston
free to be moved in a proximal direction.
In the above description the mutual engaging threads of the piston
rod and the piston are provided as an outer thread at the end of
the piston rod which engage an inner thread in a bore in the
piston. Alternatively the piston can be provided with a threaded
stud engaging an inner thread in a bore in the end of the piston
rod.
* * * * *